The use of variable-output air separation apparatuses is tending to increase, because of the advantages of standardization that they afford: the design costs are amortized over more apparatuses, the grouping of the procurements makes it possible to reduce their cost, the mass-production effect decreases the unit manufacturing cost of the apparatuses, it is possible to stock the apparatuses in order to reduce the delivery time to customers since it is known that they can meet a wide range of demands, the maintenance operations are simplified and the stock of spare parts reduced, etc.
However, variable-output separation apparatuses pose problems when adapting the apparatus itself and its feed compressor to variable operating conditions, so as to maintain for as long as possible the separation performance and the specific energy consumption.
Thus, it is known to control small air distillation apparatuses on an on/off basis: they stop when the nitrogen pressure in the user network increases up to a predetermined high threshold and restart when this pressure comes back down to a predetermined low threshold.
The drawback of this method is that the distillation apparatus takes a great deal of time to restart and, throughout the duration of restarting its compressor is consuming its nominal energy with no nitrogen being produced. If it is desired to reduce the number of stop/restart cycles, a buffer tank of large volume must be installed.
It is also known to vent the excess nitrogen output. The specific energy is then, of course, increased accordingly.